Parabolic reflective solar heater

ABSTRACT

This product is a solar cooker that combines portability with the precision of a mathematically-designed parabolic reflector. It comprises a rigid framework and a semi-rigid reflector surface. The framework has a cross-shaped footprint. The upper surface of the framework is a parabolic dish roughly two feet in diameter. The contour of the dish may be a simple paraboloid, compound paraboloid, or semi-compound paraboloid. The framework is lightweight and may be hollow and/or deconstructible for ease of portability. An alternative embodiment allows the dish to incline at various angles. The reflector surface is formed from sheets that fit atop the framework and assume the shape of the parabolic dish. Securing means on the upper surface of the framework hold the reflector sheets in place in a precisely parabolic configuration.

1. FIELD OF THE INVENTION

This invention is in the field of solar heaters, specificallyparaboloidal reflective solar heaters and cookers.

2. BACKGROUND AND OBJECTS OF THE INVENTION

In the context of this invention, the term “solar” refers to the directuse of sunlight to heat objects. It does not suggest the use ofphotovoltaic panels to generate electricity.

Solar cooking can be accomplished either by trapping sun heat in anenclosure, much like an oven, or by focusing it in one spot, much like aflame grill. In practice, it is often done by a combination of these twomethods. The present invention focuses sunlight by means of a reflectivesurface.

The geometry of a reflective solar cooker is crucial for itseffectiveness. A cooker in the shape of a parabolic dish has theproperty of focusing all incoming sunlight to one focal point. It isthis concentration of energy that allows for temperatures much higherthan the ambient environment. When a vessel of food is placed at thefocal point, it can be cooked properly.

Although several solar cookers on the market achieve a nearly perfectparabolic shape, they do so only by a relatively complicated scaffoldmade from metal tubes or wood. This makes them heavy, cumbersome, anddifficult to assemble. On the other hand, more lightweight and portablecookers only achieve approximations of a parabolic shape, leading to ahighly decreased efficiency due to the inevitable scatter of sunlight.

The present invention overcomes the disadvantages of both kinds ofcookers, achieving greater efficiency, portability and ease of assembly.The device can be carried in a hiking backpack. Using only a smallnumber of simple parts, it can be assembled in minutes, and it is verysturdy.

To generalize the scope of this invention, it is not restricted to thepurpose of cooking. Solar heaters can be used for a variety of purposes,such as boiling water, melting wax, or starting a fire. I envisioncooking as the best mode of this invention, so I will usually refer toit as a “cooker.”

3. DESCRIPTION OF RELATED TECHNOLOGY

It is well known that a mirror in the shape of a paraboloid (parabolicdish) will reflect incoming sunlight to a focus. The concentrated solarenergy can be used to heat materials or to cook food at that focus. Tobe effective, a solar cooker must have a parabolic dish about two feetin diameter or larger. The typical parabolic dish is made and sold inone piece. Such a product is bulky and can be heavy. It is difficult totake a parabolic dish on a camping or hiking trip.

The reflective surface of a parabolic dish can be a lightweight sheet offlexible material such as aluminum foil, vinyl, or Mylar. The“Copenhagen Cooker,” invented by Sharon Bush-Clausson, makes use of foursquare vinyl panels and a central cardboard base plate. Each panel istied to one edge of the base plate with bootlaces. The panels are thencurved upward and clipped together with clothespins. The result is abowl-shaped reflective surface. The Copenhagen Cooker is extremelylightweight and portable. Its limitation is that its shape is only arough approximation of a paraboloid. It is not nearly as effective as asolidly constructed reflector.

One challenge associated with solar cooking is the fairly limited rangeof directionality. In a typical reflector, the image of the sun willcross the entire cooking vessel in an hour, whereas effective cookingcould take several hours. The reflector must be continually monitoredand adjusted. Variances in season and latitude are secondary concerns.The sun's height in the sky is very different in the summer than in thewinter. The sun is also higher in the sky at lower latitudes. Areflector that is designed for use in the tropics may be inefficient intemperate climates.

The directionality problems may be mitigated with a compound paraboloiddesign. A compound paraboloid is formed by two parabolas with the samefocus and co-planar axes of symmetry. Each parabola is revolved aroundits axis of symmetry. The intersection of their solids of revolution isthe compound paraboloid. Stated heuristically, the compound paraboloidis formed by “half of one paraboloid and half of another,” with a cusppoint at the boundary. As the sun moves across the sky, its energy willbe directly incident first on one half of the reflector and then theother. The energy is always directed to the same focus. This allows thereflector to track the sun for a greater duration of time with no needfor manual adjustments.

The compound paraboloid shape of solar reflector was first introduced inU.S. Pat. No. 4,002,499, invented by Winston and assigned to the UnitedStates Energy Research and Development Administration (the ERDA Patent).The ERDA Patent was intended primarily for the industrial-scalecollection of solar energy for purposes of providing power. It includesan “energy receiver” for absorbing the sunlight. My invention does notinclude an energy receiver. It directs energy to a container of food orother material provided by the individual using the device.

U.S. Pat. No. 4,741,610 (Dudley) discloses a compound parabolicreflector in which each surface is part of a parabolic cylinder, ieparabolic along one axis. The device utilizes two reflective surfacesthat are not contiguous. The surfaces face each other, supported in avery particular arrangement by a rigid framework. One surface reflectsenergy onto the other surface, which then focuses the radiation to apoint. The solar cooker constructed according to these principles isshown in FIG. 3 of the Dudley patent.

The disadvantage to the ERPA and Dudley patents is that they are large,heavy, bulky products. They are not portable, thus making themunavailable to hikers and campers.

What is needed is a lightweight, portable solar cooker with a preciseparaboloid or compound-paraboloid configuration.

4. SUMMARY OF THE INVENTION

My cooker consists of two main elements: a rigid framework and a set offlexible reflector sheets.

The rigid framework has a cross-shaped base. The rigid framework may beone solid piece. Alternatively, it may be formed by fitting together 2-4spines. The advantage to the multiple-spine construction is portability.

The upper surface of the rigid framework has the shape of a parabolicdish. Its design is computer-assisted, in order to achieve a veryprecise parabolic profile. The rigid framework may be mathematicallydesigned to form a simple paraboloid or a compound paraboloid, asdescribed in the previous section.

When the cooker is set up for use, the flexible sheets are set atop thisdish so that they assume the contour of a parabolic mirror. The uppersurface of the rigid framework has supporting structures to hold thereflector sheets in place. A cooking vessel support may then be situatedon the structure.

Symmetry of the design allows the reflector sheets to variously form aperfect paraboloid, a compound paraboloid, or even a semi-compoundparaboloid (that is, forming a parabola on only one of the two axes, inorder to either increase the angle of incoming sunlight, or the time ofexposure). Given the modular design of the cooker, a semi-compoundparabola can easily replace the standard parabola within less than aminute, should the weather or the type of cooking require it.

An alternative embodiment of the design allows the cooker to be tipped,so that it can face the sun at a variety of angles of inclination.

5. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the rigid framework.

FIG. 2 is a perspective view of the rigid framework in the embodimentwhen the rigid framework consists of two spines. The upper spine isshown sliding downward onto the lower spine.

FIG. 3 is a close-up view of the central upper surface of the rigidframework.

FIG. 4 shows the reflector sheets positioned on the rigid framework toform a parabolic dish. The cooking vessel support is also shown in thisfigure.

FIG. 5 is close-up view of the reflector sheets fitted into the uppersurface of the rigid framework.

FIG. 6 demonstrates the geometry of a compound parabola.

FIG. 7 is a perspective view of an embodiment of the solar cooker withvariable inclination.

FIG. 8 is a side-plan view of an embodiment of the solar cooker withvariable inclination.

FIG. 9 shows the cooking vessel support.

6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Terms in quotation marks are terms of art in this patent application.They are to be constructed in the claims as defined or used in thespecification.

My cooker consists of two main elements: a rigid “framework” (11) and aset of flexible “reflector sheets” (14).

The framework can be made from aluminum, plastic, reconstituted wood/MDFor even cell foam. It is lightweight, with portability in mind. Theframework can be made hollow and designed to hold sand or water toprovide stability while the cooker is in use.

The framework has a cross-shaped base. It may be constructed of onesolid piece. In the best embodiment, it is formed by fitting togethertwo or more “spines.” For example, in FIG. 2, an “upper spine” (17) isshown sliding downward to interlock with a “lower spine” (18). Theadvantage of the multiple-spine construction is portability. Whenseparated, the two spines are less bulky and easier to carry than aunitary dish.

The upper surface of the rigid framework has the shape of a parabolicdish. Its design is computer-assisted, in order to achieve a veryprecise parabolic profile. The rigid framework may be mathematicallydesigned to form a simple paraboloid or a compound paraboloid, asdescribed previously.

The reflector sheets are backed with a semi-rigid material such as heavypaper/light cardboard or vinyl. They are coated on the front side with ahighly reflective surface such as aluminum or Mylar.

When the cooker is set up for use, the flexible reflector sheets (14)are set atop the parabolic dish so that the reflector sheets assume thecontour of a parabolic mirror (see FIG. 4). Greater precision is furtherachieved by using “securing means” to secure the reflector sheets to theparaboloidal upper surface of the framework. Suitable examples ofsecuring means include a “ridge” (12) down the middle of each spine, ora precise line of protruding “studs” (13) that ensure the correctlateral tension of the reflector sheets. FIG. 3 shows how each ridge(12) has a slightly “elevated lip” above the upper surface of the rigidframework. In FIG. 5, the reflector sheets (14) are fitted between theupper surface of the framework and the elevated lips of the ridges. Thistight fit keeps the reflector sheets firmly in place. FIG. 5 also showsstuds (13) fitted through small holes in the reflector sheets (14),again to anchor the reflector sheets in place. The use of studs invarious locations maintains a tension in the reflector sheets. Thishelps the reflector sheets maintain their proper parabolic shape.

When the cooker is assembled, it will appear as in FIG. 4. A “cookingvessel support” (15) is attached to the surface of the rigid framework.The cooking vessel is then set atop the cooking vessel support. Althoughthe reflector sheets may not cover all area of the surface beneath thecooking vessel, that central region is in the cooking vessel's shadow,so it does not receive any incoming sunlight anyway.

To allow the cooker to fully face the sun at various angles, the rigidframework can be modified to the configuration shown in FIGS. 7 and 8.In this configuration, the spines are rounded at the bottom. One of thespines has a full-length base. The other spine is V-shaped. Theframework (11) can then be tipped to almost any inclination. It issupported by one or more wedges (16). This configuration allows easyadjustment to match the altitude of the sun in different locations andat different times of the day and the year.

In the variable-inclination configuration of FIGS. 7 and 8, the cookercan also be rotated more easily because it has less surface area incontact with the ground. Rotary adjustments can be further facilitatedby installing wheels on the cooker.

FIG. 6 demonstrates the geometry of a compound parabola. On the left,two-dimensional parabola (a) is shown intersecting with parabola (b).The two parabolas have different axes of symmetry. Each parabola is mosteffective at focusing incoming sunlight from near the direction of itsaxis of symmetry. The two parabolas have a common focal point (f). Angle(t) is the planar region between their axes of symmetry. The figure onthe right shows the common interior of the two parabolas, which is boundby curve (c). When each parabola is rotated about its axis of symmetry,the result is a three-dimensional compound paraboloid. When thereflector surface assumes the shape of a compound paraboloid, it iseffective at focusing sunlight incoming from anywhere within angle (t).As the sun's position in the sky progresses through angle (t), itremains concentrated near focus (f). This allows the cooker to beeffective 2-3 times as long as a simple paraboloidal reflector withoutbeing manually adjusted.

The symmetry and modularity of the cooker design allow the variabilityto form a perfect paraboloid, a compound paraboloid, or even asemi-compound paraboloid (that is, forming a parabola on only one of thetwo axes, in order to either increase the angle of incoming sunlight, orthe time of exposure). A semi-compound parabola can easily replace thestandard parabola within less than a minute, should the weather or thetype of cooking require it.

I claim:
 1. A portable reflective solar heater, comprising: alightweight rigid framework with a cross-shaped base and a paraboloidalupper surface with diameter between 12 and 36 inches; reflector sheetsshaped and sized to fit onto the dish-shaped upper surface of theframework, said reflector sheets having a highly reflective frontsurface and a semi-rigid backing material; securing means to secure thereflector sheets tightly against the upper surface of the framework; 2.The solar heater as disclosed in claim 1, wherein the paraboloidal uppersurface of the framework has a shape selected from the set of simpleparaboloid, compound paraboloid, and semi-compound paraboloid.
 3. Thesolar heater as disclosed in claim 1, wherein said securing meansinclude at least one of the following structures: ridges runningradially along the upper surface of the framework, so that the edges ofthe reflector sheets can be secured beneath the elevated lips of theridges; studs protruding from the upper surface of the framework, whichcan fit through small holes in the reflector sheets and maintain tensionin the sheets.
 4. The solar heater as disclosed in claim 1, wherein theframework is hollow and can be filled with sand or water to keep thecooker steady when in use.
 5. The solar heater as disclosed in claim 1,wherein the framework comprises two spines that fit together at rightangles and can be easily detached for ease of portability.
 6. The solarheater as disclosed in claim 5, wherein one of the spines has afull-length horizontal base to make full contact with the ground; theother spine is V-shaped so that the cooker may be tipped to face the sunat any inclination; each spine is slightly rounded at bottom to allowfor easy tipping of the framework; one or more wedges is included withthe cooker, to support the V-shaped spine so that it can maintain aconstant inclination.
 7. The solar heater as disclosed in claim 1,further comprising a cooking vessel support mounted on the framework andpositioned at the focal point of the paraboloid.
 8. The solar heater asdisclosed in claim 7, wherein the paraboloidal upper surface of theframework has a shape selected from the set of simple paraboloid,compound paraboloid, and semi-compound paraboloid.
 9. The solar heateras disclosed in claim 7, wherein said securing means include at leastone of the following structures: ridges running radially along the uppersurface of the framework, so that the edges of the reflector sheets canbe secured beneath the elevated lips of the ridges; studs protrudingfrom the upper surface of the framework, which can fit through smallholes in the reflector sheets and maintain tension in the sheets. 10.The solar heater as disclosed in claim 7, wherein the framework ishollow and can be filled with sand or water to keep the cooker steadywhen in use.
 11. The solar heater as disclosed in claim 7, wherein theframework comprises two spines that fit together at right angles and canbe easily detached for ease of portability.
 12. The solar heater asdisclosed in claim 11, wherein one of the spines has a full-lengthhorizontal base to make full contact with the ground; the other spine isV-shaped so that the cooker may be tipped to face the sun at anyinclination; each spine is slightly rounded at bottom to allow for easytipping of the framework; one or more wedges is included with thecooker, to support the V-shaped spine so that it can maintain a constantinclination.